Access to a wireless communications channel may be via controlled or contention access. In a controlled access system, such as in a cellular or WiMAX (IEEE 802.16) system, devices are generally allocated transmit opportunities that do not conflict with transmissions from other devices. (Some exceptions may exist, as when a device initially enters the network.). In a contention access system, devices transmit at locally determined times, and the possibility of collisions and data loss exits. Examples of such systems are Ethernet (IEEE 802.3) and Wi-Fi (IEEE 802.11).
Collisions are detrimental to the operation of the network, as they result in increased data latency and decreased system throughput (due to retransmissions) and potential data loss. Collisions are difficult to avoid in contention access-based communication systems. Existing communication protocols, such as those named above, attempt to avoid collisions. Devices operating in such systems attempt to receive from the medium before transmitting, and only proceed with transmission if the channel is found to be idle. However, this technique is not error-proof. For example, when a device A is transmitting on the medium, if a device B decides to transmit at the same instant as device A, the two transmissions will collide, typically preventing other receivers from correctly interpreting either transmission. Even if device B's transmission does not occur at the exact instant as device A's, delays in the system (e.g., propagation, processing, and/or receive/transmit switching delays) will prevent the second device from detecting a transmission that begins shortly before its own, again resulting in a collision.
Contention access systems may attempt to recover from data lost through collisions. In this case, when a collision is detected, the transmitting device waits for some time (the “backoff time”) and retransmits the data. To guard against a repeating series of collisions, the backoff time may be randomized over a time span called the “contention window.”
The above recovery mechanism is problematic when multicast or broadcast transmissions are used. Unlike unicast transmissions, which can benefit from acknowledgements or similar feedback mechanisms, multicast transmissions typically have no feedback from the receiver to the transmitter. Thus the transmitter may have no way to recognize the occurrence of a collision.
The optimal contention window size depends on the number of devices attempting to transmit on the channel. If there are few devices, a short contention window (CW) allows all devices to transmit with small chance of collisions. If more devices are attempting to transmit, the optimal contention window size is longer, to spread transmissions over a longer time and thus reduce the probability of collisions. This is illustrated in FIG. 1, where channel throughput/efficiency curves for three different contention window sizes are shown. The small CW performs best at light channel loading, and the large CW performs best at high channel load. At the lowest ends of the curves, where CW is larger than optimal, there is poor throughput performance because the channel is idle during much of the contention window. At the highest ends of the curves, where CW is smaller than optimal, channel performance is poor because the higher number of transmission attempts in the contention window results additional collisions.
Existing algorithms attempt to optimize performance by dynamically adjusting the CW size. For example, IEEE 802.11 uses an exponential backoff. The CW starts at a small value, CWmin. As long as transmissions are successful (as determined for example, be the receipt of an acknowledgement), CW remains at the CWmin value. However, upon a transmission failure, CW is doubled in size. Another failure will cause another doubling, until a predefined maximum value, CWmax, is reached. This increases CW size under the assumption that collisions result from the presence of a larger network load, and that the system will perform more efficiently with the larger CW, as described above. Any successful transmission (including a multicast transmission, with or without a collision) causes CW to revert to the small CWmin value.
Additional refinements may exist in traditional contention schemes. For example, transmissions of varying priority levels may be assigned different contention window sizes to provide the higher priority traffic with a higher likelihood of transmission.
When traditional contention access techniques are applied to a discontinuous channel, performance degradation in the form of increased collisions may result. A discontinuous channel is one where access is limited to periodic or semi-periodic intervals. For example, in a WAVE system, control and safety information is exchanged on a control or control/safety information (CSI) channel during a “CSI channel interval” which occurs for example 50 ms out of every 100 ms. During the other 50 ms, the “service channel interval,” devices may tune to other radio channels for other types of communications exchanges, and no control information is exchanged. Any control information arriving for transmission during the service channel interval must be queued for transmission during the next CSI channel interval. Likewise, service traffic arriving during the CSI channel interval must be queued until the next service channel interval. Since there is a higher than average probability that multiple devices have queued traffic for transmission at the beginning of the channel interval, traditional contention techniques result in poorer system performance when operating over discontinuous channels, since the CW size must adapt over time to the increased channel load found at the start of the channel interval. Therefore, there is a need for a more effective wireless communication channel, in which the contention windows can dynamically be changed at the start of a discontinuous channel interval based on predicted channel behavior.